1. Field of the Invention
The present invention relates to digital cameras (e.g., a digital still camera, DSC) or a mobile phone camera and other image capturing devices, and more particularly, to a method and apparatus for processing a Bayer pattern color digital image signal output from a single sensor (e.g., CMOS image sensor, CIS) within such an image capturing device.
2. Description of the Related Art
In a digital camera (also called a digicam, or digital still camera e.g., DSC) or a mobile phone camera, in order to reduce the cost and to simplify the circuit design, a single CMOS image sensor (CIS) is used to obtain pixel data in a Bayer pattern as shown in FIG. 1. When the Bayer pattern is used, only the intensity data for any one color among green (G), blue (B), and red (R) is sensed, captured or output (or displayed) by one pixel element. Color imaging with a single detector requires the use of a Color Filter Array (CFA) which covers the CMOS pixel array. The recovery of full-color images from a CMOS pixel array requires a method of calculating values of the other color intensities at each pixel. These methods are commonly referred as color interpolation or color demosaicing algorithms. Accordingly, pixel data for the remaining 2 colors for any one (center) pixel is obtained by interpolation of color intensity data of pixels neighboring the center pixel.
The prior art interpolation method using pixel data neighboring a center pixel in order to obtain the data for the remaining two colors except the data for the center pixel in a Bayer pattern is well disclosed in U.S. Pat. No. 5,506,619, and also well disclosed in Korean Pat. KR2001-56442 which improves the U.S. method. See also, U.S. Pat. No. 3,971,065 titled “Color imaging array”, issued to Bryce E. Bayer.
However, since these prior art interpolation methods are implemented by using a low pass filter (LPF) having a linear characteristic, they do not sufficiently compensate for aliasing along an “edge” (e.g., a sharp color boundary) within an image. Thus, when it is assumed that signal G in an arbitrary image has a distribution expressed by a polynomial as shown in FIG. 2, if a LPF having a linear characteristic is used for interpolation, G′3 on a line segment (interpolation), between G2 and G4, is generated by interpolation by the data of pixels (e.g., G2 and G4) neighboring (centered around) a center pixel (R in FIG. 2). As a result, the interpolated pixel data indicated by G′3 has a difference of ‘Δ’ from the actual pixel data G3 such that it causes aliasing along an “edge” within an image.